This invention relates to an electronic tuner of a type used in programmable channel selecting apparatus and, more particularly, to a method of and apparatus for automatically fine tuning such an electronic tuner to a selected frequency without pulling in an unwanted frequency.
Electronic tuners, such as a tuner used to select a particular channel in a television receiver or to select a particular station in, for example, an FM radio receiver, are known wherein a voltage-controlled variable reactance element, such as a varactor or variable capacitance diode, is supplied with a selected control voltage. Such tuning element exhibits a particular reactance representing the channel or station to which the tuner is to be tuned in accordance with the magnitude of the control voltage applied thereto. For example, the higher the magnitude of the control voltage, the higher the frequency to which the tuner is tuned.
An electronic tuner with such a voltage-controlled tuning element can, advantageously, be subjected to an automatic fine tuning (AFT) operation. Typically, this operation is performed by detecting the actual frequency to which the electronic tuner then is tuned, sensing the deviation between this instantaneous frequency and a predetermined frequency, and then adjusting the control voltage applied to the voltage-controlled tuning element in a direction to pull in the predetermined frequency. There are, however, some disadvantages attending the use of an AFT-controlled electronic tuner in a television receiver. For example, if the tuning condition of a television receiver is to be changed from a channel having a lower carrier frequency, such as channel 2, to a channel having a higher carrier frequency, such as channel 5, there is the possibility that as the magnitude of the applied control voltage increases, the AFT operation may pull in an unwanted channel whose carrier frequency lies between the lower and higher channel frequencies. That is, as the control voltage magnitude increases, it may, at some point, have an instantaneous magnitude corresponding to channel 3 or channel 4, and the AFT operation may pull in this channel. Similarly, if the television receiver is pre-conditioned to be tuned to a predetermined channel when the power supply for the electronic tuner first is energized, the AFT operation may pull in an unwanted channel, or tune the tuner to an unwanted frequency, while the power supply operating voltage, and thus the tuner-controlling voltage, increases. This possibility of pulling in an erroneous frequency is further complicated by the fact that, in typical television signal transmissions, the video information for a particular channel is modulated onto one carrier frequency and the audio information for that same channel is modulated onto a higher carrier frequency. Consequently, the AFT operation may lock the electronic tuner onto the sound information carrier frequency of an adjacent, lower channel.
It has been proposed that this erroneous pull-in result can be avoided by completely suppressing or stopping the AFT operation for a certain duration of time after the tuner power supply first is energized or after a new channel-frequency is selected, as described in U.S. Pat. No. 3,806,817. However, since the AFT operation generally is a slow, or low frequency, technique, it is preferred not to suppress or stop it, especially when the AFT circuit is used with a programmable type of electronic tuner, such as that disclosed in copending application Ser. No. 716,655, filed Aug. 23, 1976, and assigned to the assignee of the present invention. In such a programmable type of electronic tuner, various representations, such as digital representations, of individual VHF and/or UHF channels are stored. A selected representation is read out of an addressed storage location and is converted into a control voltage for application to the voltage-controlled tuning element. Nevertheless, in this type of electronic tuner, if the AFT operation is performed at a low rate, an unusually long time may be required for satisfactory operation to be completed; whereas if it is performed at a high rate, the AFT operation may pull in an unwanted frequency as the control voltage level gradually increases.